Magneto-optical overwriting methods using erasing pulses, each of which has a higher frequency and narrower width than that of the overwriting pulses

ABSTRACT

A method of overwriting information in a magneto-optical memory medium which includes a simultaneous writing and erasing step of continuously irradiating a written information-carrying area of the recording layer of the memory medium with two different pulses from a single laser beam head in accordance with information to be written, regardless of the presence or absence of written information. A variation includes irradiating the written information-carrying area with an erasing pulse from a laser beam head, regardless of the presence or absence of written information, and the then selectively irradiating the information-erased area with a writing pulse from another laser beam head in accordance with information to be written. According to these methods, it becomes possible to carry out a simple and high-speed overwriting with an increased and stabilized bit positioning accuracy.

This application is a continuation, of application Ser. No. 07/264,906,filed Dec. 31, 1988, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a magneto-optical recording method.More particularly, it relates to a method for overwriting information ina magneto-optical memory medium such as magneto-optical disks. Anoverwriting of information is a rewriting of information or data alreadywritten in areas or tracks of the recording layer of the medium.According to the method of the present invention, new information can beoverwritten on information already written in the recording layer by arelatively simple and easy process and at a high speed.

2. Description of the Related Art

Due to their excellent recording characteristics, such as high recordingdensity and large memory capacity, rewritable or overwritablemagneto-optical disks are widely used as document files and other memorymedia. Generally, the magneto-optical disks comprise a substrate such asglass or a plastic resin coated with a layer of magneto-opticalrecording material such as a metal alloy.

In the prior art magneto-optical disks, the writing, reading and erasingof information are generally carried out as follows:

WRITING OR RECORDING INFORMATION

As illustrated in FIG. 1(A), the prior art magneto-optical disk devicecomprises a disk 1, a magnet 2 used to provide an external bias magneticfield, and a lens 4 providing a focused laser beam. To simplifyunderstanding of the magnetization in the recording layer of the disk 1,the disk 1 is illustrated as a single layer.

During operation, first the directions of the magnetization in therecording layer of the disk 1 are adjusted to be in the same direction(see, arrows "a" in disk 1 of FIG. 1(B)). A high power writing laserbeam 3, as shown in FIG. 1(A), is irradiated onto a selected area of therecording layer to heat that area to a temperature higher than the Curietemperature or compensation temperature of the material constituting therecording layer. Because of the presence of an external bias magneticfield produced by the magnet 2, the heated area of the recording layerchanges its direction of magnetization to that of the external biasmagnetic field (see, arrow "b" in the disk 1 of FIG. 1(A)), and thus twodirections of magnetization "a" and "b" are generated in the disk 1.Accordingly, the information or data is written as binary signals "0"and "1" corresponding to a downward magnetization "a" and an upwardmagnetization "b", respectively.

READING INFORMATION

The written information "0" and "1" can be read by irradiating theinformation-carrying disk with a lower power reading laser beam. Whenthe laser beam is reflected on the surface of the disk, a magneto-opticKerr effect occurs in the reflected laser beam, due to the abovemagnetization. Thus, a polarization of the reflected beam is changed,i.e., a plane polarization of the beam is rotated. This rotation of thepolarization plane is determined by a detecting device to thereby readthe written information "0" and "1". The detecting device is, forexample, a photodetector provided in the reading head.

ERASING INFORMATION

Erasing can be carried out, for example, as shown in FIG. 1(B). Namely,a high power erasing laser beam 3 is irradiated onto the disk 1, and asa result, the direction of magnetization in the disk 1 is changed asshown by the arrows "a". At this stage, the magnet 2 exhibits anexternal bias magnetic field opposite to that exhibited by the magnetduring writing. As can be seen, the magnetization direction is now thesame as that at the initial stage, and therefore, the erasing ofinformation is also referred to herein as "initialization".

The above-described magneto-optical recording method is disadvantageousin that, since the magnetization direction of the disk can be changedonly in the area onto which a high power laser beam is irradiated, torewrite written information or data, the old data must be first erasedand then new data written in the same area of the recording layer. As iseasily understood, this means that at least two revolutions of the diskare necessary to carry out such rewriting or overwriting in the priorart magneto-optical disks. Also, it should be noted that the writingspeed is greatly reduced due to this excessive rotation of the disk.

To obtain a high-speed writing of information, several overwritingmethods have been suggested. For example, Japanese Unexamined PatentPublication (Kokai) No. 59-113506 teaches a magneto-optical rewritingmethod in which the power of laser beam to be irradiated onto aninformation-carrying area of the recording medium (consisting of amagnetic coating having a perpendicular magnetic anisotropy), ismodified depending upon new information to be written. JapaneseUnexamined Patent Publication (Kokai) No. 59-113507 teaches a similarrewriting method but, in this method, an external bias magnetic field ofa constant strength is additionally applied to the recording mediumduring rewriting. The application of this external bias magnetic fieldis particularly effective in relaxing the restrictions on the magneticcoating to be used. In addition, Appl. Phys. Lett.,49 (8) 473 (1986),which will be described hereinafter with reference to FIG. 2, alsoteaches a similar overwriting method utilizing a demagnetizing of themagnetic coating as the recording medium.

In the overwriting method of Appl. Phys. Lett., new data to be writtenis first compared with the old data written on the recording medium.Then, based on the results of comparison, the new data is written on themedium after an exact positioning of laser beams for writing anderasing. More particularly, the recording medium is first sequentiallychecked or observed to ascertain whether or not an (old) remainingrecorded magnetic domain which corresponds to old data or a signal "1"is present in a recording track. If a recorded magnetic domain exists,and if a signal "1" is not required in the new data, an erasing beam isthen focused on a center portion of the remaining recorded magneticdomain to remove same. Conversely, if a signal "1" is required, theerasing beam is not irradiated, and the remaining recorded magneticdomain showing the signal "1" is maintained. On the other hand, when anold recorded magnetic domain does not exist, and if a signal "1" is nolonger required in the new data, no action is taken. If the signal "1"is to be written, a writing beam is then focused on a selected site toform a new recorded magnetic domain corresponding to the signal "1".

The above-described overwriting method is illustrated briefly in FIG. 2.As shown in FIG. 2, a line (A) shows old data or information signals "0"and "1" on the recording medium, a line (B) shows a distribution ofrecorded magnetic domain (old recorded magnetic domains) formed on therecording medium in conformity with the signal "1" of the old data line(A), a line (C) shows new data to be written on the recording medium inplace of the old data of the line (A), a line (D) shows a pulse patternof a laser beam applied to the recording medium to overwrite the olddata of the line (A), and a line (E) shows a distribution of newrecorded magnetic domains formed on the recording medium by theirradiation of the pulsed laser beam. Note, the above explanation alsois applicable to FIGS. 3, 4, 6, 8, and 9. To simplify understanding, theold data in the line (A) and the new data in the line (C) are the samein all of the drawings.

Referring now to FIG. 2, it can be seen that, to erase the old signal"1" or recorded magnetic domain, a weak erasing pulse E, which isstronger than a reading power level R, is irradiated onto a centerportion of the recorded magnetic domain. Note, during overwriting, noreading is conducted and therefore the power level R in FIG. 2 isintended to explain a power level of the erasing pulse E compared withthat of the power level for reading. However, the above action is nottaken when such an old signal is to be replaced by new signal "1" in thenew data. Namely, to write a new signal "1", a stronger writing pulse Wis irradiated onto a predetermined site of the track of the recordingmedium to form a new recorded magnetic domain. In this overwritingmethod, the presence of the old signal "1" or recorded magnetic domainis ascertained by scanning another laser beam before using the laserbeam for writing and erasing. Since the erasing pulse is concentrated onthe center portion of the recorded magnetic domain, another small bubbleis produced in a central portion of the already-made bubble, and therecorded magnetic domain is erased by the interaction between thesebubbles.

The prior art overwriting methods described in Japanese Kokai Nos.59-113506 and 59-113507, and in Appl. Phys. Lett., and discussed abovewith reference to FIG. 2, increase the speed of overwriting in themagneto-optical disks, but have a drawback in that, prior to anoverwriting or rewriting of information, the position of old informationalready written in the disk must be exactly detected. This detection ofold information is cumbersome and time-consuming, and if an error occursin this detection, such as a shift in the position of the oldinformation, it is practically impossible to carry out a correctoverwriting.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved methodwhich enables an easy, simple and high-speed overwriting of informationto be carried out in a magneto-optical memory medium such as amagneto-optical disk.

Another object of the present invention is to increase and stabilize theaccuracy of positioning bits or recorded magnetic domains.

Thus, according to the present invention, there is provided a method foroverwriting information in a magneto-optical memory medium whichcomprises a substrate having a layer of magneto-optical recordingmaterial having a perpendicular magnetizing characteristic coatedthereon. The method includes a simultaneous writing and erasing step ofcontinuously irradiating a written information-carrying area of saidrecording layer with two different pulses of a laser beam from a singlebeam head, in accordance with information to be newly written,regardless of the presence or absence of written information in thearea. One of said pulses has a power sufficient to carry out a writingoperation and another one of the pulses has a power sufficient to erasewritten information, but not sufficient to carry out a writingoperation, and is irradiated onto a whole portion of the area other thanthe portion in which information is to be written.

According to the present invention, there is also provided a method foroverwriting information in a magneto-optical memory medium whichcomprises a substrate having a layer of magneto-optical recordingmaterial having a perpendicular magnetizing characteristic coatedthereon. The method includes a first step of an overall irradiation of awritten information-carrying area of the recording layer with an erasingpulse of a laser beam from a beam head, regardless of the presence orabsence of written information in the area. The erasing pulse has apower sufficient to erase written information but not sufficient tocarry out a writing operation. A second step selectively irradiates theinformation-erased area of the recording layer with a writing pulse of alaser beam from another beam head in accordance with information to bewritten; the writing pulse has sufficient power to carry out a writingoperation. Preferably, a frequency of the erasing pulse used is higherthan that of the writing pulse. This overwriting method uses an opticalhead with two beams, and therefore, is called a two-beam overwritemethod.

The magneto-optical overwriting methods of the present invention will bedescribed hereinafter, in detail, with reference to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and B is a schematic view of the writing and erasing stages ina prior art magneto-optical overwriting method;

FIGS. 2(A-E) are a schematic view of an overwriting mechanism used in aprior art magneto-optical overwriting method;

FIGS. 3(A-E) are a schematic view of an overwriting mechanism used in apreferred embodiment of the present invention;

FIGS. 4(A-E) are a schematic view of a modification of the overwritingmechanism of FIG. 3;

FIG. 5 is a graph of the recording characteristic observed in theoverwriting mechanism of FIG. 3;

FIGS. 6(A-E) are a schematic view of an overwriting mechanism used inanother preferred embodiment of the present invention;

FIG. 7 is a graph of the ability to repeat overwriting of theoverwriting mechanism in FIG. 6;

FIGS. 8(A-E) are a schematic view of an overwriting mechanism used instill another preferred embodiment of the present invention;

FIGS. 9(A-E) are a schematic view of a modification of the overwritingmechanism of FIG. 8;

FIG. 10 is a cross-sectional view of the magneto-optical disk usable inthe present invention;

FIG. 11 is a cross-sectional view of a modification of themagneto-optical disk of FIG. 10;

FIG. 12 is a schematic view of an optical head usable in the presentinvention;

FIGS. 13A to 13C are schematic views of patterns of the recordedinformation before and after overwriting;

FIG. 14 is a graph of an error rate after overwriting; and,

FIG. 15 is a graph showing changes in a C/N and error rate with anincrease of the number of overwriting operations.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As described above, the magneto-optical overwriting method according tothe present invention comprises a simultaneous writing and erasing stepof continuously irradiating a written information-carrying area of therecording layer of the memory medium with two different pulses from asingle laser beam head in accordance with the information to be written,regardless of the presence or absence of written information. An erasinglaser pulse is continuously irradiated between the irradiation of twowriting laser pulses.

According to a preferred embodiment of the present method, the erasingpulse is a series of pulses which fill in the gaps between the twowriting pulses, each of the series erasing pulses having a pulse widthsmaller than that of the writing pulses.

The above method is illustrated in FIG. 3, in which the lines (A), (B),(C) and (E) correspond to the lines (A), (B), (C) and (E) of FIG. 2. Theline (D), showing the pattern of the applied laser pulses, indicatesthat a writing laser pulse W is irradiated onto a site in which the newdata or signal "1" is to be written, to form a new pit and an erasingslim laser pulse E is continuously irradiated, having a small pulsedistance, on the recording area of the disk other than the sites onwhich new recorded magnetic domains are formed.

According to this overwrite method, since a plurality of narrow-widthpulses are irradiated as the erasing pulse, a complete erasing of oldrecorded magnetic domains can be carried out, regardless of the presenceor absence of old recorded magnetic domains in the recording track. Inaddition, since a writing pulse is irradiated to form a recordedmagnetic domain at a predetermined site of the recording track, newinformation can be directly overwritten in the recording trackregardless of the distribution of old recorded magnetic domains. Thus,the prior art problem of adjustment of the position of this writingpulse to that of the preceding pulse, for detecting old recordedmagnetic domains, does not arise.

The inventors found that to obtain a reliable and complete erasure ofold data, the frequency of the erasing pulse is preferably higher thanthat of the writing pulse used to form new recorded magnetic domainsconforming with the new data or signal "1". Preferably, the frequency ofthe erasing pulse is 3 times or more higher than a maximum frequency ofthe writing pulse. When, an erasing pulse having such a higher frequencyis used, at least one pulse will strike the recorded magnetic domain tobe removed, to reproduce a new signal "0".

In this overwrite method, the height of the erasing and writing pulsesis not critical. Preferably, as shown in FIG. 3, the height of theerasing pulses E is less than that of the writing pulses W, or as shownin FIG. 4, the height of the erasing pulses E is the same as that of thewriting pulses W. Note, although not shown, the height of the erasingpulses E may be higher than that of the writing pulses W if this doesnot adversely affect the operation.

According to another preferred embodiment of the present method, theerasing pulse is a continuous laser beam having a power stronger thanthat of a reading beam used and sufficient to erase written information,but not sufficient to carry out a writing operation. This embodiment isexemplified in FIG. 6. As shown in FIG. 6, the lines (A), (B), (C) and(E) correspond to the lines (A), (B), (C) and (E) of FIGS. 2 to 4. Asshown by the line (D) of FIG. 6, a writing pulse W is irradiated onto arecording site of the track to write information therein. A continuouserasing pulse, or preferably a continuous beam E, is irradiated onto anon-recording site of the track to remove old recorded magnetic domainsand maintain the signal "0" if no old recorded magnetic domains exist.As can be seen, the writing pulse W is a single discontinuous pulse, butthe erasing pulse E is a continuous pulse beam between the writingpulse. Further, as in the method of FIGS. 3 and 4, the frequency of theerasing pulse E is higher than that of the writing pulse W, andpreferably the frequency of the erasing pulse E is 3 times or morehigher than a maximum frequency of the writing pulse W.

Referring again to FIG. 6, a pulsed beam W having sufficient power towrite information is irradiated onto a site where new information ordata, i.e., signal "1", is to be written. This irradiation of the pulsedbeam W is carried out, regardless of the presence or absence of oldinformation or pits at that site.

According to the recording method of the present invention, informationis written or erased in the recording medium using a demagnetizationphenomenon thereof in the absence of an external magnetic field (whichmay be present if a greater stabilization of the overwriting step isrequired, or for other purposes). Therefore, when the pulsed beam havinga rewritable power is focused on a site of the recording track of themedium, two processes are carried out depending on the presence orabsence of old information or recorded magnetic domains at that site. Ifold recorded magnetic domains do not exist, the pulsed beam will reversethe magnetic field in the irradiated site by demagnetization. Thisproduces a new recorded magnetic domain at that site. If an old recordedmagnetic domain does exist, the pulsed beam will not reverse themagnetic field in the irradiated site. Namely, an old recorded magneticdomain found before the overwriting is retained as a new recordedmagnetic domain.

As shown in FIG. 6, an erasing pulse E in the form of a continuous laserbeam, is irradiated at a constant power level onto the recording track,except for the information recording site. The power of this erasingpulse E is sufficient to erase the old written information, but notsufficient to write new information, i.e., the pulse E is weaker thanthe pulse W. Note, although the pulse E has a power greater than that ofa reading power level R, this does not have any practical effect on thewritten information. The power level R is indicated herein forcomparison.

As described above, the erasing mechanism of the present method is basedon the use of a continuous laser beam having an intermediate power, asdefined herein, as an erasing pulse. The inventors found that if thelaser beam used has a suitable intermediate power, when such a beam isused in the form of a continuous beam (not a pulsed beam) it will erasewritten information but will not cause undesirable writing in anon-recorded area of the track. That is, it is possible to erase writteninformation by continuously irradiating a laser beam having apredetermined intermediate power onto the information-carrying track. Asis obvious, this eliminates the difficult operation of exactly focusingthe erasing beam in a control portion of the old recorded magneticdomain to be removed. Note, in the method of the above-discussed Appl.Phys. Lett., a small beam is selectively irradiated only onto a controlportion of the old recorded magnetic domain to be removed, but such aselective irradiation is very difficult, and therefore, is not widelyused.

In the magneto-optical overwriting methods of the present invention, therecording medium is first initialized to forcibly magnetize thedirections of magnetization of the medium in one direction, and afterinformation is first written in the initialized medium, rewriting oroverwriting is subsequently carried out. In principle, the overwritingof information is endlessly repeated once the initial writing iscompleted, but the initialization may be repeated at any stage of theworking of the medium, if desired.

The preferred embodiments of the present invention as described above,teach that, according to the present invention, a plurality of narrowpulses are continuously applied (this application is not limited tosites carrying information to be erased), and therefore, the informationcan be erased regardless of the positioning accuracy. These embodimentsalso teach that a laser beam having a weak power can be applied to eraseinformation without focusing the beam on a central portion of aninformation-carrying recorded magnetic domain, and that if the power ofthis beam is increased, the beam can be used to write new information.Accordingly, overwriting can be easily and simply carried out at a highspeed.

In addition to the one-beam overwrite method described above, thepresent invention provides a two-beam overwrite method which comprises acombination of a first step in which the written information-carryingarea is irradiated overall with an erasing pulse from a laser beam head,regardless of the presence or absence of written information, and asecond step in which the information-erased area is selectivelyirradiated with a writing pulse from another laser beam head inaccordance with the information to be written. Note, the frequency ofthe erasing pulse is higher than that of the writing pulse.

In the overwriting method of the present invention, the erasing pulsehas a frequency higher than that of the writing pulse. Preferably, theerasing pulse has a frequency 3 times or more higher than a maximumfrequency of the writing pulse.

According to a preferred embodiment of the present method, the erasingpulse comprises a series of pulses. Each pulse in the series of pulseshas a width narrower than that of the writing pulse. This embodiment isexemplified in FIG. 8, which is substantially the same as FIG. 3 exceptfor the addition of lines (D₁) and (D₂).

According to the illustrated embodiment, as shown in line (D₁), a narrowerasing pulse E as a first beam is continuously irradiated with a smalldistance between the pulses. Next, as shown in line (D₂), a writingpulse W is irradiated only on sites where information or a signal "1" isto be recorded.

As is easily understood, a reliable erasure of the information isobtained regardless of the presence or absence of old recorded magneticdomains, since a plurality of narrow pulses as the erasing beam arefirst irradiated along the track. After the removal of the old recordedmagnetic domains, a writing pulse is selectively irradiated onto therecording sites to directly overwrite or produce new recorded magneticdomains regardless of the old recorded magnetic domains and thedistribution thereof. In this overwrite process, the control ofirradiation positions, essential in the prior art process, becomesunnecessary.

Alternatively, according to another preferred embodiment of the presentmethod, the erasing pulse is a continuous laser beam having a powerstronger than that of a reading beam, and sufficient to erase writteninformation, but not sufficient for writing. This embodiment isillustrated in FIG. 9, which is identical to FIG. 8 except that thepattern of the erasing laser pulse is changed.

As shown in line (D₁) of FIG. 9, the erasing pulse E is a continuouslight beam, not a series of narrow pulses. This light beam has anintermediate power, and although it is not a pulsed laser, cancompletely erase written information, but it can not write newinformation.

After erasure of the old information, as shown in line (D₂) of FIG. 9,the writing pulse W is irradiated on sites of the track which correspondto new data or a signal "1" of the line (C) to write new information andthus produce new recorded magnetic domains. The writing pulse W usedherein has a power sufficient to write information.

The magneto-optical overwriting methods according to the presentinvention can use any magneto-optical memory medium such as amagneto-optical disk well-known in the art. The magneto-optical memorymedium generally comprises a substrate or support such as glass, aplastic, a metal or alloy thereof, a semiconductor and the like. Thesubstrate has at least a layer of magneto-optical recording materialhaving a perpendicular magnetizing characteristic, such as a metals oralloy thereof, coated thereon.

For a further understanding of the present invention, two examples ofmagneto-optical disks are illustrated in FIGS. 10 and 11.

In FIG. 10, a magneto-optical disk 10 comprises a glass substrate 11coated with, in this order, a pregrooved pattern layer 12 and amagneto-optical recording layer 13. The layer 12 with pregroovedpatterns is made of a UV-hardenable resin (so-called "2P resin" orphotopolymer) or an organic resin such as poly (methyl methacrylate),polycarbonate, and the like. The recording layer 13 is preferably madeof an amorphous alloy of a rare earth metal-transition metal such asTbFeCo, GdFeCo, GdTbFe, GdDyFeCo and the like. These alloys haveexcellent recording characteristics, can be coated by, for example,sputtering or vacuum deposition, and can provide a uniform coating overa large area. In addition to these alloys, other recording materials,for example, polycrystalline materials such as MnBi, MnCuBi and thelike, or single crystal materials such as garnet and the like also maybe used.

The magneto-optical disk of FIG. 10 may further include additionallayers as shown in FIG. 11. For example, to protect the recording layer13 from the affects of water or monomers in the underlying pregroovedpattern layer 12, a protective coating 14, usually called a primarycoat, may be inserted between the layers 12 and 13. The protectivecoating 14 may be formed from materials such as SiO₂, TiO₂, ZnS, AlN andSi₃ N₄. Further, to protect the recording layer 13 from the influence ofwater and other contaminants in the atmosphere, a protective overcoat 15may be applied to a surface thereof. The overcoat 15 also may be formedfrom any material selected from the above-exemplified materials for thecoating 14. Furthermore, the overcoat 15 may contain a lubricantimpregnated therein to prevent damage to the disk 10 due to contact withan outer magnet for producing a magnetic field.

In the present invention, an optical head illustrated in FIG. 12 may beused. In this connection, it should be noted that although theillustrated optical head is designed for a two-beam overwrite method,the optical head can be easily modified to be used in a one-beamoverwrite method.

In FIG. 12, the magneto-optical disk 10 is rotated by a motor (notshown). The illustrated optical head is used in the two-beam overwritemethod, and therefore, is provided with two semiconductor lasers 16 and23. The first laser 16 emits a laser beam of 830 nm, and the secondlaser 23 emits a laser beam of 780 nm. An optical system including thefirst laser 16 and an optical system including the second laser 23 maybe used for writing information and reading information, respectively,and vice versa.

A laser beam from the first laser 16 is transmitted to a collimator lens17 upon receipt of an input signal, where it is changed to parallelbeam. The parallel laser beam is sent to a polarization beam splitter 18and is changed to circularly polarized light by a quarter wave plate 19mounted therein. The light allowed to pass through the quarter waveplate 19 is reflected at a filter 20, passed through an objective lens21, and finally focused on a recording layer (not shown) of the disk 10.The diameter of the spot produced by the focused light is less than 1μm. Thereafter, reflected light is passed, in this order, through theobjective lens 21, the filter 20, the quarter wave plate 19, and thebeam splitter 18, and reaches a photodetector 22.

Another laser beam from the second laser 23 is first sent to acollimator lens 24, where the laser beam is changed to a parallel beam.The beam is then reflected at a half mirror 25, and the reflected beampassed through the filter 20 and focused in the objective lens 21. Thefocused beam is then irradiated onto the recording layer of the disk 10.A diameter of the spot produced is less than 1 μm, as in the opticalsystem described above. Also, a distance between the beam spot from thefirst laser 16 and that from the second laser 23 is approximately 10 to50 μm.

The beam reflected from the recording layer, as shown, is passed throughthe objective lens 21, filter 20 and half mirror 25, in that order, andthen sent to a polarization beam splitter 27 provided with a half waveplate 26. Here, to improve the S/N ratio of the signals, the beam isseparated into two components; a first component being sent to aphotodetector 28, and a second component being sent to a photodetector29. The signals from these photodetectors are combined in andifferentiator 30, which then emits output signals. The optical headhaving this construction has an advantage in that two laser beams can befocused on the recording layer, closely adjacent to each other.

Although not illustrated, the optical head may contain a servo mechanismor system for controlling the movement of the objective lens. Further,it may contain a means for producing the external bias magnetic field,if desired.

The present invention will be described hereinafter with reference toexamples thereof.

EXAMPLE 1

This is an example of the overwrite method shown in FIG. 3.

A cleaned and grooved glass substrate was sputtered to form a recordinglayer of Tb₂₄ Fe₆₈ Co₈ having a thickness of 100 nm.

Information was recorded or written in the initialized magneto-opticaldisk under the following conditions: distance between recorded magneticdomains: 2 μm; no external bias magnetic field; rotation speed of disk:10 m/sec; writing power: 5 mW. A C/N ratio of 43 dB was obtained.

Next, new information having a pit size of 1 μm and a distance betweenrecorded magnetic domains of 3 μm was overwritten on the informationwritten in the above step. During this overwriting method, short laserpulses having a pulse width (pit length) of 0.2 μm as the erasing pulse,were repeatedly irradiated between the writing pulses. A satisfactoryoverwriting was attained with good results, i.e., a C/N ratio of 45 dB,an attenuation of recorded magnetic domain signals (as described above,and a recorded magnetic domain length or distance of 2 μm) of -34 dB.

FIG. 5 is a graph plotting the recording characteristic obtained. Asolid curve I indicates the results for a recorded magnetic domainlength of 1 μm (frequency of 2.4 MHz) and a dotted line curve II showsthe results for a recorded magnetic domain length of 0.4 μm (frequencyof 12 MHz). The curve II shows that writing cannot be carried out at awrite power of approximately 4.8 mW or less, but that writteninformation is erased at that power.

EXAMPLE 2

This is an example of the overwrite method shown in FIG. 6.

A cleaned and grooved glass substrate was sputtered to form a recordinglayer of Tb₂₄ Fe₆₈ Co₈ having a thickness of 100 nm, and a protectiveovercoat of Si₃ N₄ was deposited on the recording layer.

Recording was carried out in accordance with the procedure described inExample 1, followed by overwriting in the manner described in line (D)of FIG. 6. That is, a writing pulse was irradiated on predeterminedsites of the recording track to form new recorded magnetic domains. Inaddition, a continuous beam or light having an intermediate powerbetween that of the writing pulse and that of the reading pulse wasirradiated onto the recording track, except for the recording sites.

The result of this overwriting method is plotted in FIG. 7, which is agraph showing the relationship between the write power and the erasepower. As seen at the left upper side area of line A, the erasing powerwas higher than the recording or writing power, and accordingly, writingcould not be carried out after the erasure. At the lower side area ofline B, generally an erasure could not be achieved as the erasure powerwas too small. Moreover, at the right side area of line C, erasing couldnot be carried out after writing, because the writing power was higherthan the erasing power. More particularly, it is considered that, in theleft upper side area of line A, the signal quality is reduced becausethe erasing power was too high and exceeded the power usually necessaryto carry out erasing. Such a high erasing power causes magnetization indifferent directions after completion of the erasing, thereby leavingportions in which the magnetization directions are the same as those forthe recording or writing. In the lower side area of line B, erasing,i.e., reversal of the magnetic field, could not be ensured due to a lowpower. In the right side area of line C, unreversed portions were leftdue to the large size of the recorded magnetic domains, and thus theseportions were not completely erased.

The results of FIG. 7 show that a simple and high-speed overwriting canbe obtained if a specific writing power is adopted and a continuous beamor light having an erasing capability and having a power weaker than thewriting power but stronger than the reading power, is used in areasother than the writing sites.

Next, overwriting was carried out under conditions derived from theresults of FIG. 7, wherein the power of the writing pulsed beam was 7mW, the power of the reading beam was 1 mW, and the power of the erasingbeam was 4 mW, respectively. The erasing beam was irradiated to fill inthe gaps between the writing sites, and a stable overwriting wascontinuously carried out.

EXAMPLE 3

This is an example of the overwrite method shown in FIG. 8.

A cleaned and grooved glass substrate was sputtered to form a recordinglayer of Tb₂₄ Fe₆₈ Co₈ having a thickness of 100 nm.

Information was recorded or written in the initialized magneto-opticaldisk under the following conditions: distance between recorded magneticdomains: 2 μm; no external bias magnetic field; rotation speed of disk:10 m/sec; writing power: 5 mW. A C/N ratio of 43 dB was obtained.

Then, overwriting was performed on the information written in the abovestep. A first step of the overwriting, as shown in the line (D₁) of FIG.8 was carried out, together with erasing. First, short laser pulses Ehaving a pulse width (recorded magnetic domain length) of 0.2 μm as theerasing pulse were repeatedly and continuously irradiated along thetrack of the recording layer, and then, in the second step of theoverwriting, new information was written as shown by the line (D₂) inFIG. 8 in which W indicates a pulsed beam for writing. The overwritingconditions were as follows: a recorded magnetic domain size 1 μm and adistance between recorded magnetic domains 3 μm. A satisfactoryoverwriting was obtained with good results; i.e., a C/N ratio of 45 dBand an attenuation of recorded magnetic domain signals (as above, arecorded magnetic domain length or distance of 2 μm) of -40 dB.

The above results were plotted to obtain a graph showing therelationship between the write power and the C/N ratio. The graph issubstantially the same as that of FIG. 5.

EXAMPLE 4

This is an example of the overwrite method shown in FIG. 9.

A cleaned and grooved glass substrate was sputtered to form a recordinglayer of Tb₂₄ Fe₆₈ Co₈ having a thickness of 100 nm; a protectiveovercoat of Si₃ N₄ was then deposited on the recording layer.

Recording was carried out in accordance with the procedure described inExample 3, followed by overwriting on the information written in theabove step. In a first step of the overwriting method, as shown by theline (D₁) in FIG. 9, a continuous beam of light E having an intermediatepower between that of the writing pulse and that of the reading pulsewas irradiated onto the recording track to completely erase the writteninformation. Then after the erasing, in a second step of the overwritingmethod, new information was written as shown in the line (D₂) of FIG. 9,in which W indicates a pulsed beam for writing. The conditions for theoverwriting were as follows: recorded magnetic domain size-1 μm, anddistance between recorded magnetic domains-3 μm. A satisfactoryoverwriting was obtained.

The results were plotted to obtain a graph showing the relationshipbetween the write power vs. erase power. The resulting graph issubstantially the same as that of FIG. 7.

Next, an overwriting method was carried out under the conditions derivedfrom the above results. The power of the writing pulsed beam was 7 mW,the power of the reading beam was 1 mW, and the power of the erasingbeam was 4 mW. The erasing beam was irradiated to fill in the gapsbetween the writing sites, and a stable overwriting operation wascontinuously carried out.

EXAMPLE 5

This is an example of the overwrite method shown in FIG. 4.

A cleaned and grooved glass substrate was sputtered to form a recordinglayer of Gd₀.25 Fe₀.70 Co₀.05 having a thickness of 100 nm.

Information was recorded or written in the initialized magneto-opticaldisk under the following conditions: distance between recorded magneticdomains: 2 μm; no external bias; magnetic field; rotation speed of disk:3 m/sec; writing power: 5 mW; and frequency of writing pulse: 0.5 MHz.The written information was then read and recorded in an oscillograph; apatterned pulse as shown in FIG. 13A was obtained.

Next, an overwriting method was carried out on the information-carryingtrack in accordance with the overwrite schedule in FIG. 13B, in whichthe f_(W) was 0.8 MHz, and the f_(E) was 5 MHz. The writing power anderasing power were both 5 mW. After the disk was rotated once, therewritten information was read and recorded in an oscillograph, and apattered pulse as shown in FIG. 13C was obtained. The pattern of FIG.13C shows that the signals of 0.5 MHz (FIG. 13A) were erased and werereplaced by new signals of 0.8 MHz recorded on the same track.

EXAMPLE 6

The procedure of Example 5 was repeated to ascertain changes in theerror rate after overwriting and changes in the C/N ratio and changes inthe error rate with an increase of number of overwrite operations.

First, the procedure of Example 5 was repeated using erasing pulseshaving different frequencies. The frequency of the writing pulse was 0.8MHz, and the frequencies of the erasing pulses were 1.6, 2.0, 2.4, 4.0,5.0, and 8.0 MHz. The results for the error rate after overwriting areplotted in FIG. 14. This graph shows that a relatively low error ratewas obtained when the ratio of erase frequency/write frequency was 3 ormore, and particularly, the error rate was saturated when the ratio was5 times or more. It is considered that, when the ratio is 5 times ormore, the resulting defects in the recording medium can affect such anerror rate.

At an erase frequency/write frequency ratio of 5, the C/N ratio anderror rate due to an increase of the number of overwrite operations wasdetermined, to ascertain the influence of such an increase on the C/Nratio and error rate. The results are plotted in FIG. 15, which showsthat no changes in the C/N ratio and error rate occurred during 50overwriting operations.

What is claimed is:
 1. A method of overwriting information onto amagneto-optical memory medium comprising the steps of:a) providing amagneto-optical memory medium having a substrate and a magneto-opticalrecording layer having a perpendicular magnetization directioncharacteristic coated thereon, the magneto-optical recording mediumhaving information tracks each of which carries previously selectivelyrecorded magnetic domains; b) irradiating pulse light beams inaccordance with information to be overwritten from a single optical headonto one information track of the information tracks, a first one of thepulse light beams including a series of pulses having a power sufficientto overwrite recorded magnetic domains and the remaining pulse lightbeam including a series of pulses having a higher frequency than that ofthe overwriting pulses and having a power sufficient to erase recordedmagnetic domains, each of the erasing pulses having a pulse widthnarrower than that of the pulse of overwriting pulses; and c)alternately irradiating the overwriting pulses and the erasing pulsesonto the track during overwriting and erasing, respectively.
 2. A methodaccording to claim 1, wherein a frequency of said first one of the pulselight beam is 3 times or more higher than a maximum frequency of theremaining pulse light beam.
 3. A method according to claim 1, whereinthe information is "0" and "1" binary signal information.
 4. A methodaccording to claim 1, wherein the height of the pulses of the remainingpulse light beam is lower than a height of the pulses of the first oneof the pulse light beams.
 5. A method according to claim 1, wherein aheight of the pulses of the remaining pulse light beam is the same asthe height of the first one of the pulse light beams.
 6. A methodaccording to claim 1, wherein overwriting of information is carried outin the presence of an external magnetic field.
 7. A method according toclaim 1, wherein the magneto-optical memory medium is a disk.
 8. Amethod of overwriting information in a magneto-optical memory mediumwhich comprises a substrate and a magneto-optical recording layer havinga perpendicular magnetization direction characteristic coated on thesubstrate and having information tracks each of which carries previouslyselectively recorded magnetic domains, said method comprising the stepsof:a) irradiating the information tracks with pulse light beams from asingle optical head in accordance with information to be overwritten, afirst one of the pulse light beams having power sufficient to overwriterecorded magnetic domains and the other ones of the pulse light beamsincluding a series of pulses having a higher frequency than that of theoverwriting pulses and having a power sufficient to erase recordedmagnetic domains, each of the erasing pulses having a pulse widthnarrower than that of each of the overwriting pulses; and b) alternatelyirradiating the overwriting pulses and erasing pulses onto theinformation tracks during overwriting and erasing, respectively.
 9. Amethod according to claim 8, wherein overwriting of the information iscarried out in the presence of an external magnetic field.
 10. A methodaccording to claim 8, wherein the magneto-optical memory medium is adisk.